feat: add _dd.p.ksr propagated tag for Knuth sampling rate#180
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Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Expand assert! macro call to multi-line format required by rustfmt. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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Looks good over all. Maybe the formatting should be a bit more defensive.
… type Return None for invalid rates (negative, >1.0, NaN, infinity) instead of producing garbage output. Update call sites to skip the tag when None. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Fix clippy::manual_range_contains lint. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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### What does this PR do? Fixes `_dd.p.ksr` (Knuth Sampling Rate) to only be set on spans when the agent has provided sampling rates via `readRatesJSON()`. Previously, ksr was unconditionally set in `prioritySampler.apply()`, including when the rate was the initial client-side default (1.0) before any agent response arrived. Also refactors `prioritySampler` to consolidate lock acquisitions: extracts `getRateLocked()` so `apply()` acquires `ps.mu.RLock` only once to read both the rate and `agentRatesLoaded`. ### Motivation Cross-language consistency: Python, Java, PHP, and other tracers only set ksr when actual agent rates or sampling rules are applied, not for the default fallback. This aligns Go with that behavior. See RFC: "Transmit Knuth sampling rate to backend" ### Additional Notes - Added `agentRatesLoaded` bool field to `prioritySampler`, set to `true` in `readRatesJSON()` - `apply()` now gates ksr behind `agentRatesLoaded` check - Extracted `getRateLocked()` to avoid double lock acquisition in `apply()` - Rule-based sampling path (`applyTraceRuleSampling` in span.go) unchanged — correctly always sets ksr - Tests added: `ksr-not-set-without-agent-rates` and `ksr-set-after-agent-rates-received` Related PRs across tracers: - Java: DataDog/dd-trace-java#10802 - .NET: DataDog/dd-trace-dotnet#8287 - Ruby: DataDog/dd-trace-rb#5436 - Node.js: DataDog/dd-trace-js#7741 - PHP: DataDog/dd-trace-php#3701 - Rust: DataDog/dd-trace-rs#180 - C++: DataDog/dd-trace-cpp#288 - System tests: DataDog/system-tests#6466 ### Reviewer's Checklist - [x] Changed code has unit tests for its functionality at or near 100% coverage. - [x] [System-Tests](https://github.com/DataDog/system-tests/) covering this feature have been added and enabled with the va.b.c-dev version tag. - [ ] There is a benchmark for any new code, or changes to existing code. - [x] If this interacts with the agent in a new way, a system test has been added. - [x] New code is free of linting errors. You can check this by running `make lint` locally. - [x] New code doesn't break existing tests. You can check this by running `make test` locally. - [ ] Add an appropriate team label so this PR gets put in the right place for the release notes. - [ ] All generated files are up to date. You can check this by running `make generate` locally. - [ ] Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild. Make sure all nested modules are up to date by running `make fix-modules` locally. Unsure? Have a question? Request a review! 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com> Co-authored-by: Dario Castañé <dario.castane@datadoghq.com> Co-authored-by: Mikayla Toffler <46911781+mtoffl01@users.noreply.github.com>
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## Summary of changes Add `_dd.p.ksr` (Knuth Sampling Rate) propagated tag to spans when sampling is applied via agent rates or trace sampling rules, per the [Transmit Knuth Sampling Rate to Backend RFC](https://docs.google.com/document/d/1Po3qtJb6PGheFeKFSUMv2pVY_y-HFAxTzNLuacCbCXY/edit). ## Reason for change The backend needs to know the exact sampling rate applied by the tracer to correctly compute effective rates during resampling (e.g., tracer 0.5 × backend 0.5 = effective 0.25). This tag enables that by propagating the rate via `x-datadog-tags` and W3C `tracestate`. ## Implementation details - Set `_dd.p.ksr` in `TraceContext.SetSamplingPriority()` for `AgentRate`, `LocalTraceSamplingRule`, `RemoteAdaptiveSamplingRule`, and `RemoteUserSamplingRule` mechanisms - Use `TryAddTag` to preserve the original rate (consistent with `AppliedSamplingRate ??= rate` semantics) - Format with `"0.######"` (up to 6 decimal digits, no trailing zeros, no scientific notation) per RFC spec - Added `.IsOptional("_dd.p.ksr")` to `SpanTagAssertion.cs` so integration test tag validators accept the new tag ## Test coverage - Unit tests in `TraceContextTests_KnuthSamplingRate.cs`: - KSR set for agent rate sampling - KSR set for trace sampling rules (local, remote adaptive, remote user) - KSR NOT set for manual, AppSec, rate limiter, or single span mechanisms - KSR preserved on subsequent sampling calls (TryAddTag semantics) - Formatting with up to 6 decimal digits (boundary values including small rates like 0.00001) - System tests in [system-tests #6466](DataDog/system-tests#6466) ## Other details Related PRs across tracers: - Java: DataDog/dd-trace-java#10802 - Ruby: DataDog/dd-trace-rb#5436 - Node.js: DataDog/dd-trace-js#7741 - PHP: DataDog/dd-trace-php#3701 - Rust: DataDog/dd-trace-rs#180 - C++: DataDog/dd-trace-cpp#288 🤖 Generated with [Claude Code](https://claude.com/claude-code) --------- Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
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What does this PR do?
Adds
_dd.p.ksr(Knuth Sampling Rate) as a propagated tag set when agent-based or rule-based sampling decisions are made. The tag is stored in spanmeta(string type) with up to 6 significant digits and no trailing zeros.format_sampling_ratenow returnsOption<String>and guards against invalid inputs (negative, >1.0, NaN, infinity), returningNoneinstead of producing garbage output.Motivation
To enable consistent sampling across tracers and backend retention filters, the backend needs to know the sampling rate applied by the tracer. Without transmitting the tracer's rate via
_dd.p.ksr, backend resampling cannot correctly compute effective rates in multi-stage sampling scenarios.See RFC: "Transmit Knuth sampling rate to backend"
Additional Notes
Key files changed:
datadog-opentelemetry/src/core/constants.rs— AddedSAMPLING_KNUTH_RATE_TAG_KEYconstantdatadog-opentelemetry/src/sampling/datadog_sampler.rs— Addedformat_sampling_rate()helper (returnsOption<String>, defensive against invalid rates) and set ksr in agent/rule sampling pathsRelated PRs across tracers:
_dd.p.ksrformatting to use 6 significant digits without trailing zeros dd-trace-cpp#288🤖 Generated with Claude Code